There are many methods that are used to determine the permeation properties of molecules through solids. These methods are based on measuring the amounts of substance that has already diffused through the solid. This diffused quantity can be determined by one of two ways. The first way the diffused quantity can be determined is by a measurement of the weight loss of the diffusing substance in a sealed container both before and after the diffusion. This first method does not yield entirely complete results because it is not particularly sensitive. The second method involves isolating and capturing the diffused substance and determining its quantity. While this method provides significantly better results than the first method, both methods are very time consuming. Because the diffusion process of molecules through solids is a very slow process, the measurement procedure by these methods can last for several days or even weeks.
Various alternatives have been taken to bypass these long measurement periods. For instance, one alternative involves using a thin membrane as a solid (such as a micro-tome section) and using a material that diffuses very quickly as the diffusing substance. However, this alternative has its difficulties since the thin membrane solid is difficult to prepare on thin-walled solids. Additionally, the substitution of a quickly diffusing material for the actual diffusing substance restricts the validity of the test.
Thus, there is a need in the art for a method to determine the permeation behavior of molecules through solids whereby the measured results are quickly obtained and are highly accurate. There is also a need in the art for a method to determine the permeation properties for foreign molecules into solids while the diffusing substance is still being diffused through the solid. There is also a need in the art for a method to determine the permeation behavior of molecules through a 2 layer solid where the thickness of each layer is unknown.